1. Field of the Invention
The present invention relates generally to a hoisting apparatus such as a chain block, and more particularly to a chain hoist with an overload prevent device.
2. Description of the Related Art
In a known hoisting apparatus, a rotational drive force induced by, e.g., a hand chain wheel is transmitted to a pressing drive member that is screwed on a driving shaft in a manner capable of advance and retreat, and a pressure receive member firmly secured to the driving shaft is pressed for rotation by the pressing drive member by way of a backstop wheel and a pair of friction members arranged on opposed sides of the backstop wheel.
A typical overload prevent device for use with such a hoisting apparatus has hitherto been constructed such that friction forces are utilized to cause sliding motions to occur between the members in case of possible suspension of an overload, to thereby prevent the rotational force induced by, e.g., the hand chain wheel from being transmitted to the pressing drive member.
In the conventional construction, e.g., a friction plate version, the pressing force adjustment for the setting of limit load was time-consuming due to the necessity to adjust the pressing force on the friction plates. Furthermore, the wear of the friction plates was inevitable and the limit load might possibly vary even in case of a small amount of wear with use. To obviate this, the pressing force had to be adjusted in an appropriate fashion.
The present invention was conceived in view of the above circumstances. It is therefore a major object of the present invention to provide a chain hoist with an overload prevent device, ensuring an easy setting of a maximum lifting load. Another object of the present invention is to provide a chain hoist free from risk of variation of the limit load in use in an overload condition.
In order to solve the above problems, according to a first aspect of the present invention there is provided a chain hoist comprising a drive shaft transmitting a rotational force to a load sheave; a pressure receive member firmly secured to the drive member; a backstop wheel rotating in one direction only relative to the drive shaft; a pressing drive member adapted to be screwed on the drive shaft in a manner allowing its advance and retreat, with the backstop wheel being sandwiched between the pressing drive member and the pressure receive member; a large-diameter boss projecting from the pressing drive member in the direction toward the axial fore-end; a small-diameter boss projecting from the large-diameter boss in the direction toward the axial fore-end, the small-diameter boss having threaded grooves formed in its peripheral surface; a rotary drive member adapted to be fitted rotatably around the large-diameter boss of the pressing drive member, the rotary drive member facing the pressing drive member in the axial direction; a plurality of, circumferentially extending lock teeth having a substantially triangular section, the lock teeth being formed on confronting faces of the pressing drive member and the rotary drive member, the lock teeth confronting each other being engaged with each other; a gently sloped face constituting each of the lock teeth, the gently sloped faces confronting each other being pressed against each other when the rotary drive member is rotated relative to the pressing drive member in the winding up direction; a steeply sloped face constituting each of the lock teeth, the steeply sloped faces confronting each other being pressed against each other when the rotary drive member is rotated relative to the pressing drive member in the winding down direction; a first engaging portion formed on the outer periphery of the rotary drive member; a driving wheel adapted to be fitted around the outer periphery of the rotary drive member, the driving wheel having a first engaged portion meshing with the first engaging portion of the rotary drive member; a second engaging portion formed on a protrusion of the large-diameter boss that protrudes from the rotary drive member in the direction toward the fore-end when the rotary drive member is fitted around the large-diameter boss; a rotation limit member having a larger external diameter than the internal diameter of the driving wheel, the outer peripheral edge of the rotation limit member serving as a restricting portion for restricting a displacement of the driving wheel in the direction toward the axial fore-end, the ration limit member having on its inner periphery a second engaged portion that is engaged with the second engaging portion at the fore-end of the large-diameter boss; biasing means adapted to be fitted around the small-diameter boss at a position closer to the axial fore-end than the rotation limit member, the biasing means providing a biasing force insufficient to cause the lock teeth of the rotary drive member to climb over the steeply sloped faces of the lock teeth of the pressing drive member when the rotary drive member is rotated in the wind-down direction upon the winding down operation, the biasing means providing a biasing force sufficient to cause the lock teeth of the rotary drive member to climb over the gently sloped faces of the lock teeth of the pressing drive member when an overload is applied to the rotary drive member upon the winding up operation; and a nut adapted to be screwed onto the small-diameter boss, for positioning the biasing means relative to the pressing drive member.
According to a second aspect of the present invention there is provided a chain hoist comprising a drive shaft transmitting a rotational force to a load sheave; a pressure receive member firmly secured to the drive member; a backstop wheel rotating in one direction only relative to the drive shaft; a pressing drive member adapted to be screwed on the drive shaft in a manner allowing its advance and retreat, with the backstop wheel being sandwiched between the pressing drive member and the pressure receive member; a large-diameter boss projecting from the pressing drive member in the direction toward the axial fore-end; a small-diameter boss projecting from the large-diameter boss in the direction toward the axial fore-end, the small-diameter boss having threaded grooves formed in its peripheral surface; a rotary drive member adapted to be fitted rotatably around the large-diameter boss of the pressing drive member; a lock teeth formation member adapted to be fitted around the large-diameter boss of the pressing drive member at a position facing the surface, toward the axial fore-end, of the rotary drive member, the lock teeth formation member being mounted thereon in an axially movable but rotationally prohibited manner; a plurality of, circumferentially extending lock teeth having a substantially triangular section, the lock teeth being formed on confronting faces of the lock teeth formation member and the rotary drive member, the lock teeth confronting each other being engaged with each other; a gently sloped face constituting each of the lock teeth, the gently sloped faces confronting each other being pressed against each other when the rotary drive member is rotated relative to the lock teeth formation member in the winding up direction; a steeply sloped face constituting each of the lock teeth, the steeply sloped faces confronting each other being pressed against each other when the rotary drive member is rotated relative to the lock teeth formation member in the winding down direction; an engagement portion formed on the outer periphery of the rotary drive member; a driving wheel adapted to be fitted around the outer periphery of the rotary drive member, the driving wheel being engaged between the engagement portion and the pressing drive member so as to restrict a displacement relative to the rotary drive member both in the direction toward the axial fore-end and in the circumferential direction; biasing means having a larger external diameter than the internal diameter of the lock teeth formation member, the outer peripheral edge of the biasing means acting as a restricting portion for restricting a displacement of the lock teeth formation member in the direction toward the axial fore-end, the biasing member being fitted around the small-diameter boss at a position closer to the axial fore-end, the biasing means providing a biasing force insufficient to cause the lock teeth of the rotary drive member to climb over the steeply sloped faces of the lock teeth of the lock teeth formation member when the rotary drive member is rotated in the wind-down direction upon the winding down operation, the biasing means providing a biasing force sufficient to cause the lock teeth of the rotary drive member to climb over the gently sloped faces of the lock teeth of the lock teeth formation member when an overload is applied to the rotary drive member upon the winding up operation; and a nut adapted to be screwed onto the small-diameter boss, for positioning the biasing means relative to the pressing drive member.
The chain hoist with an overload prevent device of the present invention ensures an easy adjustment of the limit load to improve the adjusting efficiency. In the event that a try to lift the load is made in an overload condition, a warning is provided in the form of noise that can be produced upon idle of the lock teeth. In addition, if the overload condition takes place, an easy lowering operation is ensured.
Moreover, when the rotary drive member is rotated in the lifting direction in the overload condition, the rotary drive member rotates while compressively deforming the biasing member and thus subjects the biasing member and the nut to a large urging force. However, by virtue of the rotation limit member intervening between the rotary drive member and the belleville spring, and by virtue of the engagement of the engaging portion of the rotation limit member with the engaged portion of the pressing drive member, the rotational force of the rotary drive member applied to the pressing drive member is by no means transmitted to the biasing member and the nut. This prevents the nut from tightening or loosening in case of rotation of the rotary drive member in the overload condition and thereby eliminates a fear that the set limit load may vary.
The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings in which the same or like parts are designated by the same or corresponding reference numerals throughout several views.